The use of aromatic and aliphatic polyisocyanates for preparing polyurethane systems by reaction with a long chain polyol followed by chain extension through a short chain polyol or short chain polyamine is well known and practiced in the industry. Aliphatic polyisocyanates are known to enhance light stability in polyurethanes; aromatic polyisocyanates tend to cause yellowing over a period of time.
There is substantial art on the subject of aromatic and cycloaliphatic isocyanates for polyurethane synthesis and the following patents are representative of the prior art:
U.S. Pat. No. 3,180,883 shows various organic isocyanates where one or more of the isocyanate groups are hindered or blocked to render it less reactive than another isocyanate group. In these compositions at least one of the isocyanates is blocked; typically both positions ortho to an isocyanate group are blocked. Examples of aromatic polyisocyanates include 3,5-dialkyl-4,4'-diisocyanato diphenylmethane; 4,4'-diisocyanato diphenyl ether; 3,3'-dimethyl-4,4'-diisocyanatobiphenyl and 3,3'- dimethyl-4,4'-diisocyanato diphenyl methane and the like.
U.S. Pat. No. 4,879,408 discloses cycloaliphatic diisocyanates in the form of isomer mixtures represented by the formula: ##STR2## wherein at least one of the isocyanate groups is blocked by an organo group in each of the ortho positions. The patent discloses in the background portion of the specification high melting aliphatic and cycloaliphatic isocyanates such as hexamethylene diisocyanate; 3,5,5-trimethyl-1-isocyanato-3-isocyanatomethylcyclohexane (isophoronediisocyanate) as being useful in preparing light-stable coating materials which are characterized with a high resistance to weathering. Examples of liquid and blocked cycloaliphatic isocyanates include 4,4'-diisocyanato-3-ethyl-5-methyldicyclohexylmethane and 4,4'-diisocyanato-3,5-dimethyldicyclohexylmethane.
British patent 1,080,739 discloses various aromatic isocyanates for use in the manufacture of polyurethane cellulosic systems wherein the isocyanate group is a hindered isocyanate group. Both mononuclear and bridged aromatic isocyanate compositions are disclosed. A methylene radical is the bridging group.
U.S. Pat. No. 4,946,925 discloses various bridged bi(cyclohexylamine) derivatives as curing agents. Each cyclohexylamine group has two alkyl groups where the alkyl groups are in the 2 and the 5 position and the amine groups are in the 4 position. Specific curing agents include 2,2',5,5'-tetramethylmethylenedi(cyclohexylamine). The tetraalkyl substitution pattern in a bridged cyclohexylamine derivative provides for extended pot life by retarding activity of the amine as a curative and the tetraalkyl substitution also enhances thermal properties of the epoxy resin. The isocyanate derivative is not disclosed.